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When someone talks about the proof of a spirit or liqueur, what's that person talking about? The proof of a spirit is measured by taking the percentage of volume of alcohol in the spirit, and doubling it. So a spirit with 44% alcohol by volume (or ABV) is an 88-proof spirit. Why does proof matter? Read on, friends.

Proof and Tax

Proof is used now primarily for tax purposes. Distillers pay taxes based on the proof gallons of their products. According to the tax code,

A proof gallon is one liquid gallon of spirits that is 50% alcohol at 60 degrees F. Distilled Spirits bottled at 80 proof (40% alcohol) would be 0.8 proof gallons per gallon of liquid. At 125 proof, a gallon of liquid would be 1.25 proof gallons.

In other words, a liquid gallon of spirit is a simple liquid measurement, equivalent to a gallon of milk. Replace that milk with 80-proof bourbon, and you have one liquid gallon of bourbon, and you have .8 proof gallons of bourbon. You'd also be cuckoo for Cocoa Puffs if you used that on your morning cereal.

A distiller who makes 1,000 liquid gallons of 80-proof bourbon would report that he or she has made 800 proof gallons of bourbon, and it's on that quantity that the government would calculate the amount of tax the distiller owes.

Truth in Labeling

As for labeling, proof statements are optional. The alcohol content must be listed, but in the format of percent-alcohol-by-volume. (Prior to 1988, alcohol content was listed in degrees of proof, but since then, the Bureau of Alcohol, Tobacco, and Firearms has required ABV statements instead.)

So, a distiller is legally required to report proof to the federal government, in the form of reporting proof gallons, but is not legally required to list the proof of a spirit on a bottle. Pick up a bottle of bourbon, and the small print on the label will say something like "43% alc/vol." A bottle might indicate the proof. Wild Turkey, for example, sells its namesake bourbon and rye at 81 and 101 proofs. But a label that proclaims the proof, in addition to the percentage, is doing so more for marketing purposes than for legal ones.

Proof: A History

The concept of "proof" is said to have started with the British Naval tradition of providing a daily "tot" or ration of rum to sailors. The man responsible for divvying up the daily ration was the purser, and many sailors were suspicious of him, assuming that the purser had siphoned off some of the rum for his own stash and topped off the barrels with water.

To ensure the rum wasn't diluted, sailors would use gunpowder to test (or "proof") the spirit. They'd take a bit of gunpowder, douse it with rum, and try to ignite it. If it caught flame, they were confident that the rum wasn't watered down.

For a rum to ignite, it needed to contain about 57% ABV. In our terms, of course, that would be a 114-proof spirit. But when the concept of proof was introduced, a 57% rum was considered 100% "proofed," so the 57% rum was defined as 100-proof rum.

Under this method of indicating proof, you would take the percentage of alcohol by volume and multiply it not by 2, as in the modern U.S. system, but by 1.75. So a 57% spirit was 100 proof. A 100% spirit would then be a 175-proof spirit (not 200 proof, as in the U.S. system.)

Scales of Proof

This scale came to be known as the Sikes scale, and it was used in the United Kingdom until 1980, when the UK switched over to labeling spirits according to their alcohol by volume.

Confusing? It gets worse.

I have an old bottle of mezcal in my possession, a gift from my in-laws. I can't find out much about it, but I was trying to find out its proof. The label reads "38º G.L." The "G.L." refers to the Gay-Lussac scale, named for a French chemist. The scale was a simple ABV measure, so in the case of my mezcal, it means the spirit was bottled at 38% alcohol by volume. The Gay-Lussac scale survives today as the OIML scale, named for the Organisation Internationale de Métrologie Légale, or International Organization of Legal Metrology.

What does this mean? It means that historically, three different scales have been used to indicate the alcoholic strength of spirits. The U.S. used the scale we're all familiar with, in which the proof number is double that of the ABV. The U.K. used the Sikes scale until 1980, and Europe and certain other countries used the Gay-Lussac scale, which is now known as the OIML scale.

Who cares, right? Well, you might, if you're reading any work of British literature written prior to 1980 that happens to mention proof. Or you might, if you happen to buy or receive an ancient bottle of some mysterious hooch.

What Does This Mean For You?

So, when you see a proof statement on a bottle, perhaps your first thought is: Yay, alcohol, let's get funked up tonight!

But let us think about proof in a different way. The proof of a spirit can also give you hints about the flavor of a spirit.

How so? First, alcohol is a solvent. As such, it carries flavor molecules to the tongue, in much the same way fat carries flavor in food. Rums, whiskeys, and gins that are higher in proof have more alcohol in them to help deliver that flavor.

The Flavor Connection

But there's another connection between proof and flavor, one I touched on last week, in my post on Tennessee whiskey. I mentioned that Jack Daniel's comes out of the barrel at approximately 125 proof and is bottled at 80 proof. What happens in between barrel and bottle to lower the proof? Dilution. Jack Daniel's, and most spirit makers, adds water to lower the proof to bottling strength. But adding water also dilutes flavor.

However, there's still another connection between alcohol level and flavor. I said that alcohol is a solvent that carries flavor to the tongue, and that's true. But it's only true as long as there is still flavor to carry. The higher the alcohol volume of a spirit, the less "room" there is in the spirit for the congeners and other molecules that add flavor.

This is why bourbon, by law, can't be distilled any higher than 80% ABV. If you go higher than that, you remove the flavor that makes bourbon bourbon. By contrast, vodka can be distilled to as much as 95% ABV before it's diluted to bottling strength, and because vodka's flavors are expected to be subtle, this is appropriate and legal.

So what do you want from a great-tasting spirit? Ideally, you'd want a spirit—a whiskey, for example—that's distilled out to a lower proof and then bottled at a higher proof. A whiskey that comes out of the barrel at 130 proof and then bottled at 100 proof will carry more whiskey character than one that comes out at 160 proof and bottled at 80.

Bottled in Bond

Sometimes, you'll see a reference to a spirit (usually whiskey) that's "bottled in bond" or "bonded." What does that mean? The concept started as a way for the U.S. government to ensure the quality of a spirit. It arose in the late 19th century, during a time when unscrupulous distillers would adulterate their product. To regain the public's trust, a group of distillers petitioned the government for the Bottled in Bond Act of 1897, which set federal standards for bonded spirits. Those standards were as follows:

The spirit must be the product of one distillation season at one distillery.

It must be aged in a federally bonded warehouse for at least four years.

Finally, it must be bottled at 100 proof (50% ABV).

"Bottled in bond" came to be known as a mark of quality, and for a time, many spirits carried it. Now, it's limited almost entirely to whiskeys, although Laird's offers a delicious bonded apple brandy.

Now, don't be confused. All bonded spirits are 100 proof. But not all 100-proof spirits are bonded. Many producers simply no longer bother with the distinction, and today, other mechanisms are in place to ensure that the spirits you buy are unadulterated, so the original purpose of the legislation has lost some bite. Nevertheless, a handful of products still carry the mark, so it's good to be aware of what it means.

About the author: Michael Dietsch approaches life with a hefty dash of bitters. He is a proud new father, boozologist, and cocktail curmudgeon. He lives in Providence. You can follow him on twitter at @dietsch.

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About the Author

Michael Dietsch is a barfly, booze hound, book hoarder, jazz fiend, and technographer. He lives with his wife, kids, and cats in Brooklyn, New York. When he's not out on the town, he's usually at home doing laundry and writing. His first book, Shrubs, was published in October 2014. He'll save your life if you ask nicely enough.

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